I have a simple little Happstack application that shows a form with an email field and a random question field to help combat spam. To get a random number I use getStdGen in my main function and pass it along to my function which creates the html. The problem is that the same StdGen is used so my random value is not random unless I restart the application.
Here's what my Main.hs looks like:
{-# LANGUAGE OverloadedStrings, ScopedTypeVariables #-}
module Main where
import Happstack.Lite
import qualified Pages.Contact as Contact
import System.Random
main :: IO ()
main = do
gen <- getStdGen
serve Nothing $ pages gen
pages :: StdGen -> ServerPart Response
pages g = msum
[ dir "contact" $ Contact.page g
... Other irrelevant pages
]
And here's the function that uses the StdGen to retrive a random question id:
getRandomQID :: StdGen -> Int
getRandomQID g =
let (rpercent, _) = random g :: (Float, StdGen)
rid = rpercent * questionsAmount
in round rid
questionsAmount :: (Num a) => a
questionsAmount = (fromIntegral . length) questions
What is the most elegant way to solve this problem?
As I wrote this question I found a solution that worked in the Happstack crash course (templates).
In your route which has a return type of ServerPart Response you can use the liftIO monad transformer to be able to perform IO actions. There's this handy function called randomRIO with generates a random Int from an input of a tuple which two Ints as range, like this:
page :: ServerPart Response
page = do
randID <- liftIO $ randomRIO (0, max)
... Code to generate response ...
where max = length questions
randomRIO can be found in System.Random and liftIO can be found in Control.Monad.Trans.